The invention described herein was made at the Lawrence Livermore Laboratory in the course of, or under, contract No. W-7405-ENG-48 with the U.S. Department of Energy.
Pulsed lasers that operate in the visible and ultraviolet region of the spectrum have potential applications in the areas of laser isotope separation and photochemistry. The rare gas halide (RGH) and rare gas excimer (RGE) lasers and the mercury halide lasers are examples of such lasers. Some of these applications require lasers that operate at high repetition rates with short pulses and modest energies per pulse. Typical parameters for such applications are energies of 0.1-1.0 Joules per pulse at repetition rates between 1 and 10 kHz, with pulse durations of 20-100 nsec. The combinations of these parameters corresponding to average power outputs of 0.1-1 kW is desirable. Lasers with suitable pulse widths and single pulse energies have been demonstrated, but they presently operate at relatively low pulse repetition rates and low average powers. These lasers are presently limited by gas heating and acoustic effects and by inadequate power conditioning systems. Higher average powers may be obtained by an increase in pulse repetition rate through application of convective flow for gas cooling, acoustic damping, and improved power conditioning techniques. The economic operation of RGH lasers at high repetition rates and high average powers is dependent upon the reliability and cost of the power conditioning system. This patent specification describes a technique for discharge laser excitation that leads to an improvement in electrical power conditioning at high repetition rates for pulsed lasers in general and RGH lasers in particular.
Pulsed RGH lasers have been excited using any of three techniques: direct electron beam excitation, electron beam-sustainer excitation and fast pulse electrical discharge. These techniques have also been used to excite a variety of other lasers (CO, CO.sub.2, Xe.sub.2, N.sub.2, etc.), and an extensive literature is available describing such excitations. General aspects of the electron beam and electron beam-sustainer approaches are discussed in U.S. Pat. No. 3,641,454 to B. Krawetz. J. Daugherty et al, in Applied Physics Letters, Vol. 28, p. 581 (1976) discuss the electron beam-sustainer approach for CO.sub.2 lasers; and beam-sustainer excitation of the Xe.sub.2 laser is described by E. Huber et al, in I.E.E.E. Journal of Quantum Electronics, Vol. QE-12, p. 353 (1976).
Electron beam excitation of RGH lasers was first discussed by Searles and Hart in Applied Physics Letters, Vol. 27, p. 243 (1975) and by Ewing and Brau in Physical Review A, Vol. 12 (1975). Mangano et al, in Applied Physics Letters, Vol. 27, p. 495 (1975) and Vol. 28, p. 724 (1976), and Vol. 29, p. 426 (1976) have described RGH laser excitation by electron beam-controlled discharge pumping, and fast pulse discharge excitation of RGH lasers was first discussed by Burnham et al, in Applied Physics Letters, Vol. 29, p. 86 (1976). Nighan, in I.E.E.E. Jour. of Quantum Electronics, Vol. QE-14, p. 714 (1978), reviews the area of electron beam-controlled RGH lasers.
These several approaches have both advantages and disadvantages in operation at high repetition rates. In the direct electron beam technique, laser excitation is produced by passage of a high voltage electron beam through a thin metal foil and through the gas, with the electron beam producing ionization that ultimately results in laser excitation. The electron beam is the only source of energy for the discharge, and the electron beam current density must be relatively high to provide sufficient power deposition to excite the laser. Electron beam current densities greater than 10 A/cm.sup.2 at voltages of a few hundred kilovolts are typically required for excitation of RGH lasers at a pressure of a few atmospheres, from the following considerations. Laser medium gain of 0.01-0.05 /cm requires power deposition of ##EQU1## where .epsilon. is the laser efficiency; and for .epsilon.=0.1 (characteristic of RGH lasers), E.sub.D =50-100 kW/cm.sup.3 requires a current density J.sub.b =10 A/cm.sup.2.
Electron current densities of this magnitude are customarily obtained with cold cathode electrode guns. Each emitting surface of the cathode is pulse charged to a voltage of the order of 300 keV, and electrons are emitted from a plasma that subsequently forms near the cathode surface. Foil heating is a major limitation on maximum pulse repetition frequency for a pure e-beam excited medium, as a representative e-beam electron beam may deposit a significant amount of its kinetic energy in the foil before entering the gas volume. Once the beam enters the target gas volume, the beam loses about 30 eV in producing an ion, and each ion has the potential to produce a 5 eV laser photon upon electron/ion combination; this represents a six-fold loss in efficiency. Further losses in efficiency occur by virtue of the tendency of noble gases, molecular halides and certain ions (Ar.sub.2, F.sup.-, etc.) produced by the discharge to absorb radiation throughout the ultraviolet. Electrons may also scatter out of the useful volume. The result is that overall efficiencies of about 6% are possible for RGH lasers pumped by electrons beams; self-absorption in RGE lasers limits the overall efficiency to .ltorsim.1%.
The use of the electron beam excitation technique for high repetition rate lasers is limited by heating of the foil window, intrinsic to this technique, since all the power input to the discharge is supplied by the electron beam. With increasing repetition rate and current density, the mean temperature of the foil increases. With present foil materials and foil cooling techniques, the average beam current density is limited to less than approximately 1 mA/cm.sup.2. For a duty cycle of 0.1%, typical for some applications of short pulse lasers, the peak current density is thereby limited by foil heating to less than approximately 1 A/cm.sup.2 under conditions suitable for efficient, high pulse repetition rate operation, which is insufficient to excite a RGH laser directly.
The electron beam current density required for laser excitation can be reduced by the use of electron beam-sustainer techniques, whereby a small, steady voltage below the breakdown voltage is applied across a pair of electrodes immersed in the same gas that is excited by the electron beam. Additional power input to the discharge is obtained from this sustainer electric field, but the sustainer current density required is apparently J.sub.B .gtorsim.2 amp/cm.sup.2 (Huber, supra).For the same power deposition in the gas, the electron beam power input can be reduced accordingly as the sustainer contribution increases. However, the electron beam power input cannot be reduced to an arbitrarily low value. Power input from the sustainer field increases with the applied electric field, but there is a maximum electric field that can be applied without the onset on ionization instabilities in the laser gas. Further, the power transfer from the sustainer field is itself determined by the electrical characteristics of the discharge produced in the laser gas by the electron beam. For RGH lasers, the electrons produced by the electron beam are rapidly removed by dissociative attachment to the halide component of the laser gas, and the electrical conductivity of the discharge is reduced proportionately. Thus, in RGH lasers, a relatively high electron beam current density is required to provide sufficient conductivity to allow adequate power transfer from the sustainer field. In a slightly different approach, sometimes referred to as the electron beam-controlled discharge, the electron density is allowed to grow by avalanche ionization from an initial value determined by the electron beam. The degree of enhancement that can be obtained over that of the electron beam alone is limited by the onset of an instability in the laser medium, leading to a constrictive arc. The reduction in electron beam current density by both these techniques is not sufficient to allow operation at high repetition rates within the limits imposed by the electron beam foil.
Simple electrical discharges are also often used to excite RGH lasers. Such discharges can produce single pulse energies of the order of one Joule and pulse lengths of 10-100 nsec, with overall efficiencies of the order of 1%.
Three methods of establishment of diffuse large area electrical discharges are available in the prior art for laser excitation. The first method uses many individually excited electrodes producing many independent small discharges in parallel. This method is typically inefficient in coupling electrical energy to the discharge and produces a markedly non-uniform spatial discharge. The second method uses a pre-ionization source such as a spark that is fired before the main discharge voltage is applied to the gas; this source produces a small density of free electrons in the gas. A main discharge voltage pulse with a fast rise rate is then applied between two large electrodes; and if the voltage pulse rise rate is sufficiently high, a diffuse and reasonably uniform discharge can be established and maintained for a short time between the electrodes. However, an undesirable collapse to a constricted arc-type discharge will follow in a few microseconds or less, if the discharge is at useful levels. If the voltage pulse rise rate is too low, a diffuse discharge does not form, and the charge flow develops in constricted channels from the beginning. The peak applied voltage for this approach must exceed the static breakdown voltage for the discharge.
A third discharge method uses an e-beam sustained electrical discharge, where the external source of ionization (the e-beam) remains on during the entire pulse. The electron beam controls the power input to the laser gas. Power is deposited in the gas only while the electron beam is applied, in contrast to the present invention where the electron beam is used merely to initiate the process. The ionization produced by this source using the sustainer method is much more intense than the pre-ionization intensity used in the uv preionization method above, and is in fact so intense that it is the dominant and controlling ionization source during the entire discharge pulse. The discharge voltage need not be high enough to produce sufficient ionization in the gas to sustain the discharge by itself. Externally sustained discharges such as these can typically be made spatially more uniform and for longer times and at higher pressures than the simpler self-sustained discharges. Only a modest fraction of the electrical excitation need be delivered by very high energy electrons passing through a foil so that maximum pulse repetition frequencies are potentially higher than for pure e-beam pumped devices. However, where a RGH laser is used, the e-beam must still supply 0.2-0.4 of the excitation energy to maintain discharge stability so that the advantages of this technique are not as great as for certain other lasers. For the 10.6 .mu.m CO.sub.2 laser, for example, the required e-beam excitation can be less than 0.1 of the total excitation energy supplied.
The techniques used to generate high current density, short pulse, electron beams are not suitable for operation at high pulse repetition rates. The electron beam is generated by application of a pulsed high voltage to a cathode, and the electron emission processes tend to destroy the cathode surface and to modify the cathode electrical characteristics. In addition, the generation of high voltage pulses requires switches capable of long life operation at high voltage and high average power. Such switches are not presently available. Substantial improvement in system performance could be made if the electron beam requirements for laser excitation are reduced. Several techniques exist for electron beam generation at lower current densities, techniques which may also allow high pulse repetition rate operation. Gridded hot cathode systems and wire-plasma electron guns are two such systems.
It is an object of the invention to provide an electrical discharge method for exciting an optical gain medium that allows high repetition rates, small initiating current densities and increased lifetimes for electrical switching apparatus.
Another object is to improve electrical discharge laser operation by reducing the peak power required to be supplied by the associated electrical circuits in exciting the laser.
Another object is to reduce the required electron beam intensity for laser operation.
Another object is to stabilize the electrical discharge at high peak powers by use of a low inductance primary power source.
Other objects and advantages of the invention will become clear by reference to the drawings and detailed description.